Thread: Oversized wiring

Recently I inspected a home with dual split system AC units. One was rated for 40 amps and the other for 25 amps. The larger AC unit had a 40 amp double breaker (Square D) and the smaller a 20 amp double breaker in the electric service panel. However both units used the same gauge copper wiring (rated for the larger unit) from the unit to the eletric service panel. I wrote it up as using 'oversized wiring' for the smaller unit. The customer service rep for the builder stopped me a week later and said they had checked with their electrician who stated that naturally, 'you cannot use undersized wiring because it will overheat - but you can use oversized wiring'. This is in AZ. If someone would confirm this statement one way or the other I'd appreciate it.

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Originally Posted by Doug Mossing

I wrote it up as using 'oversized wiring' for the smaller unit. The customer service rep for the builder stopped me a week later and said they had checked with their electrician who stated that naturally, 'you cannot use undersized wiring because it will overheat - but you can use oversized wiring'.

Regardless of where you are,you cannot "oversize" wire something.

Well, with one exception ... if the conductor is too large for the rated and allowed conductor sizes for a terminal, then oversize is bad, because you would need to cut some strands off, and that is a supper no-no.

Over fusing -yah
Over size conductors - ynet.

Doug look at it this way...

If the breaker is oversized, this means one thing-- it can allow the conductor to overheat prior to tripping. The breaker is designed to trip at a certan temperature, which will not be met because it is oversized for the smaller conductor. Overheating-- not good!!

If the conductor is oversized, the breaker will reach it's tripping pont (well it's designed to anyway) before the conductor fries (theoritically).

As Jerry stated, the one thing you can't do is use an oversized conductor and trim back the strands just to make it fit the lug-- just bad business...